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Let R and S be two relations with the following schemas.
R(P. Q, R1, R2, R3)
S (P. Q, S1, S2)
Where {P, Q}is the key for both schemas. Which of the following queries are equivalent?
1. Πp (R ⋈ S)
2. Πp (R) ⋈ Πp (S)
3. Πp ( ΠP , Q (R) ∩ ΠP , Q (S) )
4. Πp ( ΠP , Q (R) ⋈ ΠP , Q (R) - ΠP , Q (S) ) )
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- 1 and 2
- 1 and 3
- 1, 2 and 3
- 1, 3 and 4
- 1 and 2
Correct Option: D
In I, Ps from natural join of R and S are selected. In III, all Ps from intersection of (P, Q) pairs present in R and S. IV is also equivalent to III because (R – (R – S)) = R I S. II is not equivalent as it may also include Ps where Qs are not same in R and S.
